1. Field of the Invention
The present invention relates to an improvement in an electrochromic device. Hereinafter, electrochromic is referred to as "EC", and an EC device is referred to as an "ECD".
2. Related Background Art
A material which undergoes reversible electrolytic oxidation or deoxidation and thereby reversibly takes on a color when subjected to a positive or negative voltage is called an electrochromic (EC) material. The color change phenomenon itself is called electrochromism. Attempts have been made for over 20 years to manufacture from such an EC material an EC device (ECD) which takes on color or returns to its transparent state by the adjustment of an applied voltage, and further to utilize such an ECD in a light control device (e.g., glare shielding mirror or a 7-segment figure display device). For example, Japanese Patent Publication No. 52-46098 proposes a solid state ECD in which a transparent electrode film (cathode), a tungsten trioxide thin film, an insulating film made of, for example, silicon dioxide, and an electrode film (anode) are formed in sequence on a glass substrate. When an electric field is applied to this ECD, the tungsten trioxide (WO.sub.3) thin film becomes blue. The color of the WO.sub.3 thin film fades away and the thin film returns to its transparent state when the polarity of the electric field is reversed. This coloring and color change reversing mechanism of the EC material has not yet been explained. However, it is believed that a small amount of water contained in WO.sub.3 and the insulating film (ion conductive layer) controls coloring and reversing of color change of WO.sub.3. The reaction formula when WO.sub.3 takes on a color is theorized as follows: ##STR1##
At least one of a pair of electrode layers that is directly or indirectly separated by an EC layer must be transparent so that coloring or disappearance of color of the EC layer can be observed therethrough. In the case of transmitting type ECD, both of the electrode layers must be transparent. Examples of transparent electrode materials are SnO.sub.2, In.sub.2 O.sub.3, ITO (a mixture of SnO.sub.2 and In.sub.2 O.sub.3), and ZnO. However, these materials do not exhibit excellent transparency, and this necessitates the electrode layers made of such materials being made thin. For this and other reasons, the ECD is generally formed on a substrate such as a glass or plastic plate.
The coloring response of the conventional ECD has heretofore proved unsatisfactory, and it is this problem which is a principal concern of the present invention.